Field of the Invention
The invention relates in general to a communication circuit and an associated calibration method, and more particularly to a communication circuit that performs a built-in calibration by an up-conversion filter in a frequency translating loop filter, and an associated calibration method.
Description of the Related Art
Various kinds of wired/wireless network systems, e.g., 2G, 3G and subsequently evolved mobile communication systems, local area network (LAN) systems, sensor networks, analog/digital audio/video broadcasting systems, and positioning systems, have become integral parts of the modern information society. In a network system, both a transmitter end and a receiver end include respective communication circuits as a network interface. A communication circuit at the transmitter end is provided with a transmitter path, which may include a digital transmission block, a digital-to-analog converter (DAC), a transmission filter, an up-conversion mixer, and a power amplifier. A communication circuit at the receiver end is provided with a receiver path, which may include an amplifier (e.g., a low-noise amplifier (LNA)), a down-conversion mixer, a reception filter, an analog-to-digital converter (ADC) and a digital reception block. To support information exchange between terminals of the network system, the communication circuits at the terminals may include transmitter paths and receiver paths for transceiving signals.
When a transmitter end is to send information (e.g., digital data, packets, commands and messages) to a receiver end, the information is encoded by the digital transmission block in the transmitter path to form a low-frequency (e.g., baseband/intermediate-frequency (IF)) digital waveform, converted to a low-frequency analog signal by the DAC, passed through the transmission filter, and modulated to a high-frequency (e.g., radio-frequency (RF)) analog signal by the up-conversion mixer. Such high-frequency analog signal is processed by the power amplifier to increase its power and then transmitted to a network medium, e.g., transmitted into space by an antenna. After being received at the receiver end, the high-frequency analog signal is amplified by the amplifier in the receiver path, demodulated into a low-frequency analog signal by the down-conversion mixer, passed through the reception filter, converted to a low-frequency digital signal by the ADC, and processed by the digital reception block to restore the original information.
In the communication circuit, as the receiver path is required to restore the original information from a weak high-frequency signal received from a remote external end, operation parameters of the receiver path need to be extremely carefully adjusted and calibrated. For example, the down-conversion mixer in the receiver path includes an in-phase mixer and a quadrature-phase mixer, whose operations are respectively controlled by corresponding bias voltages. These two bias voltages need to be calibrated to overcome non-linearity caused by mismatch between the in-phase mixer and the quadrature-phase mixer. Further, the LNA in the receiver path may include a load, e.g., an inductor and/or a capacitor. A resonance frequency of the load affects a frequency response of the LNA, and thus also needs to be calibrated to ensure that the resonance frequency of the LNA satisfies an expected standard. Further, a passband frequency of the reception filter, e.g., a frequency range of a 3 dB bandwidth, also requires calibration.
In the prior art, operation parameters of a communication circuit integrated into a chip are calibrated by an external test machine during a factory test procedure. When calibrating the receiver path, the test machine feeds a test tone to the receiver path to simulate an external signal that the receiver end receives, and determines whether the operation parameters of the receiver path are appropriate according to responses of the receiver path with respect to test tone to accordingly calibrate the receiver path.
The prior art is not only costly but also time-consuming. Further, the above conventional solution can only be implemented during the factory test procedure. Once a chip is installed and operated, no calibration can be performed in response to performance drifts caused by temperature or supply voltage variations.